The present invention is generally related to a method of converting an anionic living end to a protected free radical “living” end in a well-controlled manner, which enables copolymerization by plural mechanisms. The method is particularly useful for copolymerization of anionically polymerizable monomers and free radical polymerizable monomers, and therefore useful for production of PB-g-PS, HIPS, ABS, TIPS and MBS etc. The present invention also provides a reversibly coupled polymer, which, when decoupled, generates “living” free radical for further copolymer chain reaction.
Copolymerization is one of the most widely used techniques in the polymer industries. For example, copolymerization of styrene and butadiene is necessary in preparing rubber modified styrenic polymers such as high impact polystyrene (HIPS), acrylonitrile/butadiene/styrene (ABS) resins, and methylmethacrylate-butadiene-styrene (MBS) resins.
ABS resin generally comprises a rigid matrix copolymer phase of styrene and acrylonitrile monomers having dispersed therein a graft copolymer of a butadiene rubber substrate grafted with the styrene/acrylonitrile copolymer. ABS resins are most often produced by initially preparing a polybutadiene seed latex and copolymerizing styrene and acrylonitrile in emulsion in the presence thereof.
Rubber-reinforced polymer compositions of the HIPS type are widely used in many applications because of their ease of molding, good gloss, and generally good mechanical properties. HIPS polymers are usually formed by incorporation of polybutadiene (PB) phase in polystyrene (PS) through the in situ formation of PB-graft-PS. Early in the polymerization, phase separation begins because of the immiscibility of the rubber within the polystyrene being formed and the depletion of the styrene phase. The immiscibility of PB in PS and the in situ formation of the compatibilizer, PB-graft-PS, give rise to the formation of varied morphologies, for example, of the lamellar, globular, capsule, and other types. These microstructures are responsible for the absorption of energy when the material is subjected to high intensity (impact) or low intensity (tension) forces.
Typically, a conventional process for the preparation of HIPS consists of polymerizing a styrene monomer (or other vinyl aromatic monomer) in the presence of an appropriate quantity of polybutadiene or butadiene copolymer. For example, such polymers can be manufactured by the polymerization of styrene in the presence of 5-20% dissolved polybutadiene or butadiene copolymer rubber.
However, the technology of converting an anionic living end to a protected free radical “living” end has not been developed. Furthermore, the technology has not been utilized in, for example, copolymerization of butadiene and styrene, or industrial production of HIPS or ABS.
It is generally accepted that polymerization with a high degree of structure control are only possible by means of relatively complex methods, such as anionic, cationic or group transfer polymerization, but the choice of monomers is limited and the reactions demonstrate a high sensitivity to impurities. Conventional free radical polymerization shows tolerance to the presence of functional groups, but provides relatively poor structural control. Controlled free radical polymerization offers an opportunity to combine the advantages of conventional free radical polymerization with those of living ionic polymerization.
The present invention advantageously provides a method of converting an anionic living end to a protected free radical living end, and therefore also provides a copolymerization method using multiple mechanisms, i.e., anionic and free radical mechanisms. One of the merits obviously associated with the method is its ability to conveniently copolymerize anionically polymerizable monomers and free radical polymerizable monomers in a controlled manner. For example, this invention can be utilized to prepare butadiene polymers that will react with styrene monomer forming butadiene-g-styrene copolymer during styrene homopolymerization.